1. Field of the Invention
The present invention relates to a device for detecting a coherent optical signal comprising means for generating an auxiliary optical signal, also coherent, and means for causing the optical signal to be detected and the auxiliary optical signal to beat and for detecting the beat and so the signal to be detected.
Such a device is used, for example, in laser telemeter systems or else in active imagery systems. In these systems, the optical signal to be detected comes from the reflection from an object, or target, of an optical signal emitted by an emitter laser. For example, in the case of telemetry, the optical signal emitted is pulsed and measurement of the time interval which separates the time of emission from the time of detection of the reflected signal makes it possible to determine the distance at which the target is situated.
The auxiliary optical signal has a frequency slightly offset with respect to the frequency of the optical signal emitted by the emitter laser, for example of the order of a few tens of MHz. It is generated by a laser which therefore plays the role of local oscillator and its amplitude and frequency characteristics are invariable in time. The optical signal to be detected and the local optical signal are applied, for example, to a quadratic detector which causes these two signals to beat. This detector is followed by a filtering circuit centred on the beat frequency, so as to permit detection of the beat and so the signal to be detected. Such detection is called coherent optical detection.
In such a device, the maximum distance at which a detectable target may be located is obviously related to the value of the weakest signal capable of being detected. In fact, as is known, the further the target is away the weaker the signal received from reflection from the target of the emitted signal. In practice, the lower limit of the detectable signal is related to the signal to noise ratio of the local oscillator laser. Therefore, to increase the maximum value of the measurable distance, an optical amplifier may be used for amplifying the signal to be detected before beating.
However, conventional optical amplifiers themselves have a signal to noise ratio such that the increase of the performances of the assembly is little significant.
2. Description of the Prior Art
However, from the article by NAKAJIMA et al. "Intracavity nearly degenerate four-wave mixing in a (GaAl) As semiconductor laser", Appl. Phys. Lett. 47(8), 15 Oct. 1985, a device is known for amplifying in particular an optical signal without appreciably degrading the signal to noise ratio. This device uses a mixing of four optical signals, which mixing is of the type usually designated by "four-wave mixing", and which takes place here in a laser diode oscillating at the frequency f.sub.1. This device provides not only, as is known, a change of sign of the phase of the optical signal which is applied thereto but also amplification of this signal. This amplifier device is however not applicable in the coherent detection device of which it is a question here, to the extent that when the signal to be amplified is at frequency f, the amplified signal is at frequency f.sub.a, with: EQU f.sub.a =2f.sub.1 -f
The frequency f.sub.a of the amplified signal is therefore equal to the frequency f of the signal to be amplified only if the latter is strictly equal to the oscillation frequency f.sub.1 of the laser diode of the amplifier device. Now, such a condition cannot be permanently obtained in the coherent detection device of which it is a question here, considering the independent variations which the two frequencies in question are likely to undergo.
Thus, the sensitivity of known coherent optical detection devices is therefore limited.